1. Field of the Invention
The present invention relates to a transceiver and communication method employing time division multiple transmission methods, which performs data transmission among a plurality of transceivers.
2. Description of the Related Art
Time division multiple transmission methods such as Time Division Multiple Access (TDMA) or Carrier Sense Multiple Access (CSMA) are technologies that multiplex signals of a plurality of channels on the time-axis by sending the signals at different times, respectively. The time division multiple transmission methods are widely used in digital telecommunication and computer networks as described in many books, for example, “OFDM Modulation Technology For Digital Broadcasting And Mobile Communication,” Makoto Itami, Triceps, 2000.
In the time division multiple communication method, transmitted data from a transmitter are divided into base units, which are called “packets”, “slots” or “frames.” Each packet of a channel is sent in proper timing by a multiplexer. In a receiver, a process that a demultiplexer picks up signals of the channel, which are required for the communication, is performed in a transceiver that works as a receiver. As shown in FIG. 10, a transceiver 500 involving one of the time division multiple communication methods includes a control circuit 501, a transmitting circuit 502, a receiving circuit 503, and a switch 504. The control circuit 501 performs digital signal processing including producing transmitting signals and demodulating receiving signals, outputs the digital transmitting signals and controls the transmitting circuit 502, the receiving circuit 503, and the switch 504. The transmitting circuit 502 converts the digital transmitting signals to analog transmitting signals, and outputs the analog transmitting signals to a transmission line 505 outside of the transceiver 500 via switch 504. The receiving circuit 503 receives analog receiving signals from the transmission line 505 outside of the transceiver 500 via the switch 504, converts the analog receiving signals to digital signals, and outputs the digital receiving signals to the control circuit 501. The switch 504 is controlled by a control signal from the control circuit 501 so that the switch 504 switches between two positions connected to the transmitting circuit 502 and the receiving circuit 503.
From an analog viewpoint, the switch 504 in the transceiver 500 switches between two positions of receiving position and transmitting position. Therefore, all of transceivers except a transceiver that works as a transmitter are under the receiving condition.
Accordingly, for example, as shown in FIG. 11, when there are five transceivers 500A, 500B, 500C, 500D, and 500E, each of which has 50 (Ω) impedance of internal transmission line and the transceiver 500A sends data to the transceiver 500B, then receiving power in the transceiver 500B will decrease by 8 dB in comparison with the situation shown in FIG. 12, in which there are only two transceivers 500A and 500B. One reason why the receiving power in the transceiver 500B will decrease by 8 dB is considered to be because each impedance of the three transceivers 500C, 500D, and 500E in disuse for the communication affects the transceiver 500B as combined impedance as these transceivers 500C, 500D, and 500E are connected in parallel to the transceiver 500B. In general, each of the transceivers 500A, 500B, 500C, 500D, and 500E may have both transmitting impedance and receiving impedance under normal conditions. However, as a matter of convenience, the receiving impedance in the transceiver 500A and the transmitting impedance in the transceivers 500B, 500C, 500D, and 500E are not shown and considered in FIGS. 11 and 12.
In particular, the combined impedance among the transceivers 500B, 500C, 500D, and 500E is 12.5 (Ω). Accordingly, receiving voltage of the transceiver 500B becomes 0.2V in light of the combined receiving impedance 12.5 (Ω) and the transmitting impedance 50 (Ω) when the transmitting voltage of the transceiver 500A is 1.0V. The receiving voltage (0.2V) in FIG. 10 decreases by 8 dB in comparison with the receiving voltage (0.5V) in FIG. 11.
The 8 dB decrease of the receive voltage leads to a 8 db decrease of the ratio of signal to noise (S/N) in transceiver 500B. Furthermore, this 8 dB S/N decrease may diminish the transmission efficiency of the transmission line by a factor of four. For example, where 16 Quadrature Amplitude Modulation (QAM) with a reception bit error rate of 10−5 may be employed in the system illustrated by FIG. 12, the 8 dB diminution of S/N in the system illustrated by FIG. 11 may restrict this system to the use of Binary Phase Shift Keying (BPSK) as a modulation/ demodulation method.
As mentioned above, one or more unused transceivers connected to a transmission line during a communication causes a decrease in receiving power in a transceiver undergoing communication when a time division multiple transmission method is used. Accordingly, connection of the unused transceiver(s) to the transmission line results in a decrease of the S/N of the transceiver in use for the communication.